Air-laydown apparatus for assembling fibers into webs

ABSTRACT

An apparatus is disclosed for high speed production of uniform webs by air-laydown of textile fibers. A feed batt of staple fibers is fed to a toothed disperser roll that projects the fibers at high velocity and low angle into an airstream of high uniform velocity and low turbulence to form a thin fiber stream from which the fibers are subsequently separated in the form of a web. The webs are suitable for producing high quality nonwoven fabrics by known fiber-interlocking or bonding treatments.

United States Patent 11 1 1111 3,906,588 Zafiroglu 1 Sept. 23, 1975 [5AIR-LAYDOWN APPARATUS FOR 2,676,364 4/1954 Plummer et a1. 19/156.4

ASSEMBLING FIBERS INTO WEBS Dimitri P. Zafiroglu, Newark, Del.

Assignee: E. I. Du Pont de Nemours and Company, Wilmington, Del.

Filed: Feb. 14, 1974 Appl. No.: 442,404

Related US. Application Data Division of Ser. No. 322,757, Jan. 11,1973, Pat. No. 3,797,074, which is a continuation-in-part of Ser. No.241,924, April 7, 1972, abandoned, and Ser. No. 135,734, April 20, 1971,abandoned, and Ser. No. 135,735, April 20, 1971', abandoned.

Inventor:

US. Cl l9/l56.3; 19/97 Int. Cl. DOIG 25/00 Field of Search 19/159,156-1564,

References Cited UNITED STATES PATENTS 4/1954 Plummer et a1. 19/156.4

FOREIGN PATENTS OR APPLICATIONS 944,854 12/1963 United Kingdom 19/1141,010,147 11/1965 United Kingdom.... 1,050,832 12/1966 United Kingdom19/156.4

Primary Examiner-Dorsey Newton [57] ABSTRACT An apparatus is disclosedfor high speed production of uniform webs by air-laydown of textilefibers. A feed batt of staple fibers is fed to a toothed disperser rollthat projects the fibers at high velocity and low angle into anairstream of high uniform velocity and low turbulence to form a thinfiber stream from which the fibers are subsequently separated in theform of a web. The webs are suitable for producing high quality nonwovenfabrics by known fiber-interlocking or bonding treatments.

7 Claims, 7 Drawing Figures Sept. 23,1975 Sheet 1 of3 "3,906,588

US Patent US Patent Sept. 23,1975 Sheet 2 0f-3 3,906,588

US Patent Sept. 23,1975 sheet3of 3 3,906,588

FIG-7 AIR-LAYDOWN APPARATUS FOR ASSEMBLING FIBERS INTO WEBS BACKGROUNDOF THE INVENTION A This invention relates to an air-laydown apparatusfor assembling textile fibers into webs, and is more particularlyconcerned with improvements in dispersing and transporting textilefibers in an air stream for collection on a moving screen to form webswhich are suitable for use in producing high quality nonwoven fabric.

Nonwoven fabrics are produced from fibrous webs by bonding orinterlocking the fibers to provide durability and strength. The fibersof the'web may be bydraulically entangled by treatment with high energyliquid streams as disclosed in Evans U.S. Pat.- No. 3,485,706, issuedDec. 23, 1969. When producing relatively heavy weight textile fabricLauterbach US. Pat. No. 2,910,763, issued Nov. 3, 1959, disclosesthatfiber interlocking may be initiated by treatment with a needle loom andcompleted. by crimping or shrinking the fibers. Production of bondednonwoven fabrics may be accomplished as disclosed in Graham US. Pat. No.2,765,247, issued? Oct. 2, 1956. The quality of fabric produced by thesemethods depends upon the quality and uniformity of the web which istreated.

Webs suitable for producing high .quality nonwoven fabrics, bytreatments of the above type, can'be prepared by air laydown of textilefibers. Prior art airlaydown processes and apparatus are illustrated byBuresh U.S. Pat. No. 2,451,915 issued Oct. :19,- 1948, Plummer et al.U.S. Pat. No. 2,676,363 issued Apr. 27, 1954, and Simison US. Pat. No.3,381,069 issued Apr.

30, 1968. Staple fibers are shipped as a compacted mass. Conventionalpicking and carding operations are used to separate the fibers.Theresulting loosely opened fiber lap is fed to a toothed disperserrolland a stream of air is sucked or blown over the roll. The roll isrotated at highispeed to feed the fibers into the air stream, theobjective being to feed individual fibers rather than clumps or groupsof fibers. The-fibers are carried by the air stream as a spreading cloudthrough a conduit to the screen surface of a condenser roll or conveyor,where the fibers are deposited over a relatively large surface area toform a layer on the moving screen. The Plummer et al. patent discussesthe importance of air turbulence for providing a generally uniformdistribution of fibers over relatively large areas throughout theconduit. Turbulence is introduced in the air supply by changing thedirection of air flow ad- 1 jacent to the disperser roll, byproviding arestricted air passage past the roll into an expanded duct, and byirregularities in the duct leading to the condenser roll.

A commercial embodiment of such prior art layer forming apparatus isshown in FlGS.- 38 and 40 of Evans US. Pat. No. 3,485,706. As stated atcolumn 20,

line 65, The layer-forming apparatus processesa whereas'the jet-treatingapparatus is capable of high speed opc rationl Non-blotchy, uniform websweighing about 1 ounce per square yard (oz/yd?) have been produced at amaximum rate of about 18 feet per minute'(ft./min. or about-0.63 poundsper inch of disperser-roll width per hour (lb/in. h.r.), when using suchlayer-forr'ning apparatus. Production of high quality webs atsignificantly higher speed does not appear to be possible unless basicmodifications are made in this layerforming apparatus.

SUMMARY OF THE INVENTION The present invention provides an air-laydownapparatus suitable for high speed production of uniform webs ofexcellent quality from feed'batts of staple fibers; The new air-'laydownapparatus makes possible the production of such webs at rates in excessof 3 pounds per inch of web width during each hour of operation, evenwhen lightweight webs of 0.5 to 2 oz./yd. are produced from fine, longstaple fibers (e.g., 1.25 denier, l /z-inch long fibers). Highly uniformwebs weighing 4 to 10 oz.yd. or more can be produced at rates of over 10lbs/in. hr. and, in some cases, at rates above 15 to 20 lbs/in. hr. Theinvention provides webs of high' quality when evaluated for blotchiness(i.c., small nonuniformities which may be formed, for example, bydepositing clumps of fibers), streakiness (i.e.. lines of differentfiber density which may be caused, for example, by local variations inair stream velocity), and other visible defects. A g

The present invention is an apparatus for assembling textile fibers intoa web which comprises duct means for conveying fibers in a controlledflow of air, fiber disperser means for projecting fibers into the ductmeansto form a thin stream of fibers in air, air supply means fordirecting a low turbulence flow of air through the duct means, andcondcnser means for separating the fibers from the air to form a web.The duct means includes sidewalls and endwalls forming a rectangularcross-section of at least the width of the web. One of the sidewalls hasan opening through which the fibers are projected. The walls aresubstantially straight and parallel. up to this opening to maintain theair in stable flow over the opening. The fiber disperser means comprisesa toothed disperser roll, suitable for rotation at a surface speed of atleast 3,000 feet per minute, and a stationary disperser plate having acurved surface spaced from the roll teeth. This spacing is less than0.125-inch from a point where the fibers are picked up by the roll teethto a point where they are projected into the air stream, to form anarrow slit where the fibers are projected through the opening into theduct by inertia.

The disperser roll may be 5 to 50 inches in diameter and have between 8and 350 teeth per square inch of surface. The teeth are usually shorterthan 0.250-inch. with 0.010 to 0.030-inch clearance between the teethand the disperser plate. Boundary layer control means in the duct meansprior to the above opening may be provided for improving the boundarylayer of air through which the fibers are projected. This control meansmay bea thin boundary obstruction extending across the duct walls toeliminate streamwise vorticity in boundary air flow. Another controlmeans is a suction slot close to the opening in the duct wall forremoving t'urbulent'boundary air flow.

The air supply means may comprise a high uniformity wind tunnel oflarger cross-sectional area than the duct means; and a flow nozzleconnection for providing a controlled flow'of substantially uniformvelocity up to where the fiber stream is formed. The duct means may havea substantially uniform rectangular cross section along the path of thefiber stream to the condenser means. or it may converge to acceleratethe air stream conveying the fibers. Preferably, the duct means issubstantially straight.

GENERAL DESCRlPTlON The fibers are preferably centrifugally doffed froma toothed disperser roll rotating at a surface speed of at least 3,000ft./min. Supply means feeds a substantially uniform layer of fibers ontothe rotating disperser roll and a closely spaced. curved disperser plateholds the fibers close to the roll until a fiber-doffing position isreached at the tip of the disperser plate. At this location the streamof fibers is projected by tangential ejection from the disperser rollthrough an opening into duct means for supporting the fibers in air. Theroll is mounted outside of and adjacent to the wall of the duct means sothat a small surface are of the roll is exposed within the duct andfills the opening without appreciably reducing the cross-sectional areadefined by the duct walls. a Air supply means directs a uniformvelocity. stable. low turbulence, low vorticity flow of air through theduct in the direction of movement of the roll surface so that the fibersare projected into the stream at an angle of less than about 25 andpreferably less than 12. This projection. angle is measured between theline tangent to the disperser roll at the point adjacent to the tip ofthe disperser plate and a line which coincides with the generaldirection of air flow through the duct. Boundary layer control means maybe incorporated in the duct upstream of the exposed roll surface toprovide a controlled. low level of turbulence in the air layer throughwhich the fibers must uniformly project to reach the region of the airstream which has uniform flow of low turbulence and low vorticity. Theduct configuration downstream 'of the disperser roll is contoured toprevent boundary layer separation and its attendant formation. ofvortices and eddies; generally sudden changes in duct cross-section ordirection are avoided. Condenser means separates the fibers from the airto form webs weighing from about 0.] to 10 oz.- /yd.". 1

A- conventional type of toothed disperser roll is suitable forprojecting the fibers into the uniform flow of air. The curved surfaceof a disperser plate is arranged at a small distance from the revolvingsurface of the roll to provide a narrow passage where the fibers arecarried to the point of projection into the duct. The spacing betweenthe stationary disperser plate and the tips of the disperser roll teeth.from the point where the fibers are picked up by the teeth to the pointwhere they are projected into the air stream. should be less thanone-eighth inch. The disperser roll teeth are usually shorter thanone-fourth inch. preferably about oneeighth inch. To the extent that thefibers penetrate between theteeth. the fiber stream projected into theair stream will be thicker than the clearance between the ends of theteeth and the disperser plate. Preferably, that clearance will be 0.010to 0.030 inch. The disperser roll has a surface speed of from .3000ft./min. to

a maximum limited-by speeds which damage the fibers or cause excessivevibration; preferably the surface speed is about 10,000 to 20.000ft./min. The roll diameter should not be excessive. since the fibers arepro- 5 jected tangentially only a'short distance before losing momentum,but a diameter of at least 16 inches may be required to reduce vibrationon a wide machine. Preferably the portion of the disperser plate facingthe roll has a low-friction surface to facilitate projection of thefibers.

The duct means for conveying fibers provides a conduit through which auniform velocity. low turbulence stream of air is passedadjaeent to theexposed surface of the disperser roll and on to the fiber condensermeans. The duet configuration is such that substantially all ofthefibers projected from the disperser roll are supported in the flow asa thin fiber stream (e.g.. ofless than one-fourth inch initialthickness, and preferably less than one-eighth inch initial thickness)which is essentially kept away from layers of high turbulence.nonuniform flow. or separated flow. For economical operation thecross-section of the duct should be the minimum necessary for carryingout the process.

Preferably, the duct has a substantially rectangular cross section. Thecross-seetionalarea may increase or decrease as the duct approaches thecondenser means. provided that the changes :in cross-section do notdisturb the fiber stream. The adverse effects of boundary layerseparation in a toorapidly-diverging duct (eg, a diffuser in which thewalls diverge at more than a 30 angle) are avoided. Channel curvaturethrough the duct should be kept to a minimum. but a slight curvature canhelp maintain the fiber stream centered in the duct. The length of theduct means should be sufficient to provide room for the variousapparatus components.

The upstream side of the duct means is connected to conventional airsupply means. such as is used in wind tunnels. for providing an air flowof low turbulence intensity and minimized eddy size. The required airsupply can be furnished by fans blowing air into a highly uniformpassage provided with flow distribution devices. such as vanes.perforated plates. honeycomb sections. and turbulence reduction screens.which also act as eddy eliminators. Typically. the air is forced througha multicell structure having uniform cells of regular crosssection. Eachcell has diagonals of about one-half inch maximum length (preferablyone-eighth inch). a maximum wall thickness of one-sixth inch (preferably0.010 inch) and a cell length of at least 25 cell widths. Otherequivalent'vortex elimination systems are known in the art. The airpassage will normally be the same width as the duct into which thefibers are projected and several times greater in the othercross-sectional dimension. Thus. the air velocity in the larger airpassage in which the screens. honeycombs and perforated plates arelocated is much less than the air velocity in the relatively small ductmeans. The transition between the large air passage and the small ductmeans is gradually curved in accordance with good flow nozzle design toaccelerate the air flow of low turbulence and substantially uniformvelocity is supplied from the nozzle to the duct means.

Of key importance to this invention is the condition of the Howimmediately upstream of the opening in the duct means through which thefibers'are projected. The portion of the air stream which advances intothe fiber path is carefully controlled. The air velocity in this regionis uniform across the thickness of the r egion as well as acrossincrements of width of the regions Across the thickness "of the region,the velocity in cach0.']- ineh-thick layer (excluding the first 0.1-inchthickness next to the wall) is within $157!,- and preferably within :1of the average velocity in the region, and is designated hereinafter asAV/V. Values of AV/V in excess of il57 result inexcessive' mixing amongthe layers which leads to eddies which disturb and expand the fiberstream.-This causes the loss of fiber stream thickness and trajectorycontrol leading to blotchiness in the web product. The velocityvariation within any one foot increment across the width of the regionof the air flow that advances into the fiber stream is less than 110%,

and preferably less than $571, and is designated hereinafter as AV/W.Values of AV/W in excess of $107: result in eddies which deleteriouslyaffect the fiber stream thickness and trajectory, thus leading toblotchiness in the web product. The average turbulence inten sity (i.e.,the standard deviation of velocity variation with time) in the region isless than 157:, preferably less than 771. These numerical values referonly to the portion of the air. stream which advances into the fiberpath, and exclude the boundary layer portion within 0.1 inch of thewall. Average turbulence intensities of greater than 15% in this region(as well as in the subsequent path of fiber flow) or large eddies, asindicated by large velocity variations and unstable local turbulenceintensities, prevent formation of a thin fiber stream, cause the fibersto disperse as an expanding cloud, and result in excessive blotchinessin the web product. The air passing through the duct means outside theregion which advances into the path of fiber flow need not be limited inturbulence intensity or velocity distribution, so long as its influenceon the air stream conveying the fibers to the condenser is such that theaverage turbulence intensity at any crosssection of the fiber path fromthe disperser to the condenser is less than 15% and preferably less than7%. Turbulence intensities in excess of l7 in this portion of the fiberpath also result in excessive blotchiness in the web product. i

It is recognized that turbulence is produced by fluid friction with thewalls of the duct. This can be confined by a boundary layer next to thewalls by use of smooth, streamlined duct walls and by supplying the airflow to the duct in a proper condition, as described in the precedingparagraphs. The duct is also arranged to keep the fiber streamessentially centered in the low turbulence, uniform flow region, so thatboundary layer turbulence will not disturb the fibers after the thinstream of fibers is formed. However, the fibers must be projectedthrough'the boundary layer turbulence on the disperser roll side of theduct. Thus, gross nonuniformities in that boundary layer should beavoided. A serioud nonuniformity problem may be caused by a phenomenonof streamwise vorticity (i.e., small corkscrewtype vortiees with highenergy). These vortiees can be detected by scanning the flow with a hotwire anemometer across a line perpendicular to the side walls of theduct in the planeof the flow cross section. A vortex shows as alocalincrease in turbulence intensity and a corresponding decrease inlocal air velocity. :Fibers projected through such vortiees. aredeflected from their intended paths. This results innonuniformdistribution of fiber in the airstream, which in turn producesstreaks in the web pI'OdLlCtw-A SUCtlOlTFSlOt can be used in theboundary layer that develops in the duct means shortly downstream of thelocation where the air enters the-ductfrom the'curved flow nozzle or inthe newly developing boundary layer downstream of a suction slot. It hasbeen found that web streaks due to these vortiees can be eliminated whena very thin obstruction (frequently called a boundary layer trip), suchas a strip of sandpaper or a smooth round wire of about l8 gauge'(about0.04 inch in diameter), is extended across the duct wall, in theboundary layer, just upstream of the laminar-to-turbulent transitionzone. The transition to turbulence is then uniformly accelerated to forma uniform, stable, turbulent boundary layer which is penetrateduniformly by the fibers. The turbulence added by these obstructions isusually small and in general does not add significantly to stream mixingover the disperse'r roll or 'to the blotchiness of the final web.- A

combination of a suction slot to remove the boundary layer followed by avery fine wire boundary layer trip to prevent nonuniform redevelopmentof the boundary layer, is preferred in cases where the slot cannot beplaced sufficiently close'to the disperser roll. because of spacelimitations.

Large random eddies, arising from lack of proper preparation of the airflow to the duct means. can also cause the fibers to disperseuncontrollably as a cloud rather than as a thin stream, as soon as thefibers leave the disperser roll. in these cases, the air turbulence at agiven point in the duct means is unstable. This instability ischaracterized by random, large, sudden increases in turbulence intensityto values of 1.5 to 2 times the average turbulence intensityat thatpoint. even though the average turbulence intensity may be well belowthe desired 15% level. These large sudden increases in turbulence causeloss of control of fiber stream thickness and trajectory. These unstableflows are usually also accompanied by large scale, relatively lowfrequency air velocity nonuniformities. In the operation of thisinvention, a stable, substantially uniform velocity across the width ofthe duct means is maintained at lines parallel to the slot where fibersare projected into the air stream. The velocity variation within any onefoot distance along these lines (excluding end or wall effects),referred to herein as AV/W, as indicated above, is smaller than il0'/(of the average within that same lfoot distance and preferably within$57!.

The fibers are conveyed to condenser means of a type conventionally usedto separate fibers from an air stream in the formof random filoer webs.The fibers are uniformly deposited in a succession of overlapping areas,somewhat analogous to the way shingles are laid. The shingle length isdirectly proportional to the width of the fiber laydown area and to theapparent thickness of the fiber stream. A.low-turbulence thin streamhaving a fixed (non-oscillating) trajectory gives a shingle length ofabout one-half inch. An increased area of fiber-laydown can be providedby placing the condenser screen at a more oblique angle to the fiberpath. Alternatively; a modification of the invention accomplishes anequivalent effect with means for controlling the flow of air to increasethe effective fiber laydown area. Mixing stream generator means forcontrollably developing gradually enlarging eddies of low turbulenceintensities can be located. in the duct meansbetween the duct entranceand the disperser roll position. For example, a three-fourth inch metalstrip perforated with oneeighth inch holes to give 42% open area, andfastened to the end walls of the duct to extendacross the centralportion of the duct, was found to increase the width of the effectivefiber-laydown area from V2 inch to about 1% inches, thus giving a spreadstreamv effect. Alter natively, circular or square rods of variousdiameters or sizes (e.g., rods of one-sixteenth to one fourth inchdiameter), depending upon. the flow-mixing and shingle length desired,can be used to provide thiseffeet. By

blocking the upper half of the flowat the nozzle delivery with a solidor perforated plate, the area of laydown can be increased even more.These mixing stream generators initiate stable eddies of largedimensions while leaving the region of the air stream'entering the fiberflow path essentially unaffected. The mixing stream generators of thisinvention produce stable average turbulence intensity values of lessthan 15%, preferably less than 7% in the-fiber flow path. By use ofmixing stream generators and long ducts (e.g., about 6 feet long) theeffective fiber laydown area can be extended to as much as about 12inches. All these mixing stream generators. cause some lateral mixingwithin the fiber stream, which may be desirable for eliminating any finestreaks in the web produced by small nonuniformities in the fiberdelivery point. These positive effects, however, must be balancedagainst the increase in blotchiness resulting from an-accompanyingincrease in turbulence caused by the mixing stream generator.

BRIEF DESCRITPION OF THE DRAWINGS dispersing section showing anotherembodiment of the invention.

FIGS. 4 and 5 are longitudinal vertical sectionsof a part of theapparatus showing other embodiments-of the invention.

FIG. 6 is a fragmentary section taken on line 66 of FIG. 4 showinganother embodiment of the invention.

FIG. 7 is a longitudinal vertical section of a part of the apparatus.showing another embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG. I,there is shown a fiber feeding means consisting, in this embodiment, ofa conveyor belt 2, feed roll 3, compressing roll 4 and shoe 5 forsupplying fiber l to the disperser roll 8. The fiber feeding means isdesigned to feed a butt of staple fibers having a weight, in ounces persquare yard, which is about 3 to 150 times theweight'of the web to beproduced. The disperser roll separates the fibers and carries them mixedwith the air adjacent to the roll surface through the space between theroll and disperser plate 10, and discharges this mixture centrifugallyinto lel walls 16 of the airpassag'e are connected to the duct walls 20by converging section 18 of the flow nozzle configuration. Screens 38and 42, and honeycomb structure 40, provide a uniform flow substantiallyfree of turbulence and vorticity. Air is blown into the air passage byone or more fans 36, through a duct system 33, shown diagrammatically.

The fibers'are'deposited to form a web on continuous, moving screen 26which is driven and supported by rolls 28'and 30. The air-flows throughthe screen and is withdrawn through vacuum duct 34. The air may befiltered to remove any particles passing screen 26 and then berecirculated to fan 36. Several fans in series or an open air systemwithone or more fans supplying the air and one or more fans exhaustingthe air can also be used. The screen 26 is sealed against the fiber duct20 and the vacuum duct 34'by sealing means 32 such as a plate ofpolyethylene. i FIG. 2 shows another embodiment of the doff-bar 12having a rounded edge 13' and an upper surface 15 which slopes. The upof the doffing bar 13 should be less than 0.125 inch from the teeth ofthe disperser roll and preferably about 0.010 to 0.015 inch away fromthe teeth. I

FIG. 2 also shows a further embodiment of the invention wherein thedisperser plate 10 is provided with a slot 44 connected by conduit 46 tovacuum manifold 48 and a vacuum pump (not shown') In the arrangementshown the walls 50 of the vacuum manifold 48 serve to support thedisperser plate 10. The slot may have awidth of from 0.02 to 0.12 inch.The suction slot 44 serves to reduce or remove completely any turbulentboundary laycrin the air strcanrthat may develop on the lower side oftheairduct. A vacuum of from 1 to 20 inches of water is suitable. i i IIn FIG. 3, dotted line 58 is the tangent to the outer edge of thedisperser ,roll teeth 7. The upper edge 54 of disperser plate 10 can beplaced on the tangent line 58 or ,can be somewhat below .the tangentline. e.g. onehalf inchbelow. Preferably thedisperser plate tip 52 isrounded with a radius of at least 0.015 inch but less than about0.06.inch. The face 56 of the disperser plate is essentially concentricwith. the disperser roll. The clearance between the face .56 and theteeth, 7 should be less than 0.125 inch in order to avoid prematureturbulent mixing of air and fiber under. the plate which would result infiber clumps. Preferably a clearance of between about 0.01 and 0.03 inchis used.

An additional embodiment of the invention is shown by boundary layertripping means 62 in FIG. 3. The tripping means consists of a very thinobstruction, such as a round wire of about 18 gauge ora 0.50-inch widestrip of 60 grit sandpaper, locatedon disperser plate top surface 54about l'to 1 0 inches downstream from the start of the duct means, asdetermined for the particular --process' conditions. The resultingturbulent boundary layer (of the order of one-fourth inch thick) isrelatively uniform ac'rossthe width of the duct.

The centerline of the fiber duct 22 may ex ten d a straight lineparallel to the top oft he disperser plate as shown in FIG. 1 or it canbe bent as shown in FIG .-4. The angle between the top of the disperserplate and the centerline of the fiber duct at the condenser section istermed the bending angle The bending angle should be less than 30 and ispreferably from to 10. FIG. 4 also illustrates reduction ofthe-cross-sectional area of the duct over the doff-bar 12 to cause theair to ac celerate and provide a flatter velocity-distribution after thefiber stream is formed. i

FIGS. 4 and 6 show a further modification with the provision of eddygenerator means 66 located in the air stream prior to the fiber doffingpoint. Generator 66 consists of a perforated plate or scfreen coveringabout the middle third of the air duct 20. The plat'e shown in FIG. 6with a staggered array of holes (e.g., 0.l2 -inch diameter) and an openarea of about 42% 'will provide a spread stream effect as fibers aredeposited on' rotary condenser screen 26. A solid platelin the upperportion of the air duct, and small diameterisolid rods located in themiddle of the air stream upstream of the fiber doffing point are alsouseful as eddy generator means FIG. 5 shows another arrangement of asuitable machine with the disperser roll named above the fiber duet.FIG. 5 also illustrates the use of a condenser screen 26 moving at a 45angle to the fiber stream.

FIG. 7 shows another embodiment which has a pair of conveyor belts 70and 71, a pair of compression rolls 72 and 73 about which the belts run,and a pairof feed folls 74 and 75. The batt of fibers l is carried bythe belts 70 and 71 between restraining means 76 and fed through the nipbetween positively-driven, fluted, feed rolls 74 and 75 to the transferzone 80 defined by the space between the feed rolls and the teeth ofdisperser roll 8. Feed roll 74 is mounted in fixed shoe 79 and feed roll75 is mounted in a spring-loaded shoe 77. The initial loose staple fiberbatt, havinga bulk density of no more than about 1 pound per cubic footand preferably weighing from 40 to 150 ounces per square :yard (ormore), is first partially compressed: between the belts,

is further compressed to maximum density of at least 4 pounds per cubicfoot in the nip ofthe feed rolls. and then expands to a density of about2 to 5 pounds per cubic foot in the transfer zone, where it is forwardedin a substantially radial direction to the toothed surface of thedisperser roll.

The disperser roll 8 is of conventional design and is usually about 5 to50 inches diameter. It is usually of hollow construction. Thecylindrical outer surface of the roll is usually provided with low rake,fine metallic wire clothing 7 (FIG. 3 formed by spirally winding one orseveral saw-tooth strips about the roll and anchoring it. The sharp endsof the teeth are located so that the ends lie in a substantially truecylinder about the axis of rotation of roll 8. Typical arrangementsinclude:

Tooth rake Face angle within about 8 from radial direction. I

Tooth length Shorter than one-fourth inch, preferably about one-eighthinch. I Tooth ends Tip width less than 0.030 inch. Tooth density Betweenabout 8 and 350' teeth per square inch of roll surface. I

Roll Diameter Peripheral Speed Acceleration (inches) (feet/minute)(times gravity) .l6 3000 to ZO OOO l 17 to 5200 24 3600 to 24.000 1 12to 5000 32 4200' to 30,000 1 l5 to 5700 90 or more of the arc of thedisperser roll. Although unitary disperser-plates and doff bars areshown in I FIGS. 1 and 2, both parts can be fabricatedofa'number ofsections with'suitable attachments.

Inallof the above-described embodiments of this in- 'vention, disperserroll 8 projects fibers into the air 'stream at an initial angle of lessthan 12 to thegeneral direetion of air flow. Although low angles offiber projection are preferred, angles as high as about 25 are suitablein some cases. At angles greater than about 25 the roll intrudesexcessively into the duct means. so

' thatunstable eddies formjust upstream of the disperser roll. Thiscauses an unstable and nonuniform region of air flow to be advanced intothe fiber stream being projected by the disperser roll which results ina thick cloud offibers, rather than a thin stream. being dispersedJThisin turn results in a blotchy. streakyweb product.Moreover,=with angles greater than 25. the fibers'may be impingedagainst the opposite duct wall leading to agglomeration of thefiberdispcrsion. and

preferred design is that of Rouse andHassan in Mechanical Engineering";'Volume: '7l. I No. 3. March. 1949, which is approximated in FIG. 1.This air supply assembly provides a uniform flow of air at the exit ofthe converging section 18. Exclusive of boundary layers (ile.. layerswithin about one-halfineh of the duct walls), the flow has a totalvelocity variation across the cross-section, both vertically andlaterally of less than 1071. but-usually and preferably less than 15%.and a stable turbulence intensity of less than about l5 /t. but usually'and preferably well below 7%. The peripheral velocity (V of thedisperser roll 'is at least 3.000

ft./min. A'suitable range of air velocity (V) at the edge of thedisperser'plat'e in Zone: A is between about 0.5 and 3.5 times Vand'preferably about 0.5 to L2 times V,,. A suitable rang'e'of maximumair velocity in Zone A (i.e., over the point ofhigh'est, intrusion ofthe disperser rollinto the duct)-is between about 0.5 and 3 times V0.but preferably between 0.7 andl .7.times V Suitable ratios of weightrates offlow of air to fiber are at least 25 to I.

Measurement of Air Velocity and Turbulence Intensity The velocity of theair flow at various points is determined with a conventional hot wireancmometer. A suitable instrument for this purpose. which was used forthe measurements reported herein. is a Model 1050 8-4 hot-wireancmometer,manufactured by Thermal Systems Inc., of St. Paul. Minn.Others are well known in the art. When the output of the anemomcter isalso passed to ana-c coupled, root-mean-squarc (RMS) voltmeter, such asa. Model 3400A, manufactured by Hewlett Packard. lnc.. of Loveland,Colo.. the RMS value of the velocity fluctuation in the direction of airflow with time is measured. For the values reported herein. the RMSreadings were averaged for about to 10 seconds. The RMS value of thevelocity fluctuation, multiplied by 100 and divided by the averagevelocity at that location is referred to herein as the local turbu lcnceintensity. Further details on the use of hot wire anemometers formeasuring velocity and turbulence.

intensity is given in numerous places in the art, such as Bulletin 53,The Hot Wire Anemomcter, of Flow Corporation of Cambridge. Mass.Theoretical discussionsof turbulence intensity are found in H.Schlichting, .Boundary Layer Theory", 6th Ed.. McGraw Hill Book Company,New York. 1968. pages 455457,

538539. 558. etc. i

The velocities and turbulence intensities which are of key importance tothis invention are those associated with the air. layers that advancealong the fiber path and carry the fibers to the condenser. To determinethe path .of the fibers in, the process of this invention is relativelysimple. since the processv provides a distinct stream of fibers. One wayis to determine the fiber path -optieally through transparent sectionsof the duct means. For convenience, the fiber stream can be lighted fromthe top and the thickness clearly seen and measured from the edge of thetransparent section of the duct means. Permanent records of the fiberpath can be made with regular speed.Polaroid" photos or high speed (nearIO second exposure) Polaroid photos. The high speed photos show theinstantaneous position of individual fibers-in the duct. Once the fiberpath has been determined. the velocity and turbulence intensitymeasurements are made with no fiber flowing. The resultant measurementswithout fiber flowing are considered typical of the case with fiberflowing (all other things being equal) because of the very high minimumweight ratio of air to fiber flow (i.e., at least to l j The velocitiesand turbulence intensities are measured at at least three typical keylocations: l) in the flow layers advancing over the disperser plate tipinto the fiber path just upstream of the disperser roll; (2) in thefiber path flow just downstream of the disperser roll over the doff bartip; and (3) in the fiber path flow at the end of the doff barjustupstream of thccondenscr. The velocity and turbulence intensities aremeasured at each of these locations, in a typical stream position (i.e..removed from the side walls) at intervals of 0.1 inch, starting 0.l inchaway from the wallof the duct containing the disperser-roll opening andcontinuing through the thickness of the fiber path. The measurements ateach location are averaged for the thickness of the path to give anaverage velocityand an average turbulence intensity. If the fiber pathis very wide,.scans of the path should be made at additional points inthe width oitbc stream at the threeabove-mcntioned locations. in allembodiments of this invention, the average turbulence intensity at eachof these locations along the fiber path is no more than about 15%Average turbulence intensities in excess of 15% result in blotchy webs.

Measurements of velcoity across the width of the fiber path at the samethree locations mentioned above (without fiber flowing also provide ameasure of large scale, low frequency, velocity nonuniformities.Immediately upstream of the disperser roll. the velocity variation in afoot width of stream advancing into the fiber path is less than il07rfor embodiments of this invention, and preferably less than i596. Thevelocity variation for 0.1 inch thick layers of this advancing stream(excluding the first 0.1 inch from the wall) is within 15% (preferablywithin 10%) of the average velocity in the region.

If the fiber path from the disperser roll to the condenser is long, morethan the three turbulence-intensity measurcing locations mentioned aboveshould be used to characterize the turbulence to which the fiber streamis exposed in its advance to the condenser. It is recommended that themeasuring locations be spaced essentially equidistant from each other.

i The following examples, which illustrate specific em bodiments of thisinvention. are not intended to limit the invention in any way.

EXAMPLE l in this example. an apparatus. similar to that illustrated inFIG. 1,. is used to form 36-inch wide, high quality webs at thefollowing rates:

Production Rates Product Weight lbs/in. hr. yards/min oz/ytl 6.5 l.] 9.178 l. I I'll (v8 1 .6

In each of the above runs. the feed to the disperser roll consists ofLZS-denier-pcr-fiber. three-fourths inchlong. polyethylene tcrephthalatestaple fibers in the form of a loosely opened l00-oz./yd. batt. This isfedto a 24-inch diameter disperser roll having 40 teeth/ square inch.each tooth being 0.09 inch high and 0.009

inch thick. The clearance between the ends of the teeth width of theduct at this location is less than 1% per foot. The approximate heightdimensions of 36-inch wide rectangular duct 20 and the average airvelocities at various locations in the duct are as follows:

Thickness Velocity Location (inches) (ft/min) X. immediately downstreamof nozzle (i.e.. at entry to rectangular duct) 2 9000 l Over plate 10just upstream oldisperser roll l 3/8 H.000

-Continued Thickness Velocity Location (inches) (ft/min) 2. At point ofmaximum intrusion of roll into duct 7 7/8 20.600 3. Over plate l2, justdownstream of disperser roll I 1/8 16.000 4. Just upstream of collectingscreen 26 l 7/8 9.600

The distance between locations X and l is about 9 inches; between 1 and3, about 8 /2 inches; and between 3 and 4, about I) inches. The fibersare projected into the duct at an initial angle to the airflow ofabout 9and then conveyed in the air stream in a thin straight path to thecollecting screen. At no location along the fiber path is the turbulenceintensity greater than about 2%.

EXAMPLE II This example illustrates the effect of turbulence intensityalong the fiber path on the production rate of high quality, defect-freewebs.

Apparatus similar to that illustrated in FIG. lis used to form a 70-inchwide web weighing 1 oz/yd at a rate of 60 yards per minute. The feedbatt consists of I.5-denier-perfiber, threc-fourth-inch rayon staplewhich is carded into a loosely opened fiber lap. This is fed at a rateof 6.25 lbs/in. hr. to a l6-inch diameter disperser roll rotating at asurface speed of 12,500 feet per minute. Air is supplied through theduct at 5,000 feet per minute under conditions such that the turbulenceintensity in the air stream leaving the nozzle is about 0.4%. A uniformweb of high quality is produced. When the turbulence intensity leavingthe nozzle is increased from 0.4 to 0.7%, equivalent blotch level in theweb can be obtained at production speeds of only about 40 yards perminute or less.

EXAMPLE III This example shows the beneficial effects that can beobtained through use of suction as a means of boundary layer control.

Apparatus similarto that illustrated in FIG. 1, but provided withsuction means for removing boundary layer" turbulence as shown in FIG.2, is used to form a 36-inch wide web weighing I oz/yd at a rate of 80yards per minute. An air-formed batt of 1.5 denier per fiber,three-fourth-ineh polyethylene terephthalate staple is fed at a rate of8.3 lb/in. hr. to a 24-inch diameter disperser rollrotating at a surfacespeed of 15,000 feet per minute. Air is supplied through the duct at8,000 feet per minute under conditions such that the maximum turbulenceintensity in the fiber path is about 0.5%. A uniform web is producedwhich is of high quality and free from streaks. When the above-describedconditions are repeated without suction to remove boundary layerturbulence, some streaks are formed due to nonuniformities in theboundary layer.

EXAMPLE IV In this example a series of webs is produced under differentconditions of turbulence intensity along the fiber path.

An apparatus similar to that illustrated in FIG. 1 is used to form aseries of l l-inch wide webs of about I oz/yd unit weight at a rate of 5lbs/in. hr. The batts fed to disperser roll 8 are oz/yd. batts of 1.25denier per fiber, three-fourths inch long polyethylene terephthalatestable fibers prepared from multiple 2 oz.yd layers formed on aRando-Webber The disperser roll is 16 inches in diameter, has teeth persquare inch, each tooth being 0.090 inch high and having a 0.009 inchtip thickness. A 0.02 inch clearance is maintained between the tips ofthe teeth and plate 10. The disperser roll projects a thin stream offiber into the duct means at an initial angle of 1 1 with the generaldirection of air flow and at an initial velocity (V,,) of I 1,600ft/min. the dimensions of the rectangular duet means at specifiedlocations downstream of the cubic nozzle are as follows:

screen 26 In this apparatus, the moving screen 26 interrupts theair-fiber stream at a 45 angle. The distance between the specifiedlocations X and l, l and 3, and 3 and 4 are respectively 8, l l and I2inches.

To control the turbulence intensity in the fiber path a series of mixingstream devices were positioned at location X. The webs produced withthese devices and under the conditions describe-d above were thenarranged in order of increasing blotchiness. The results are summarizedin the table below. The designations V V V and V, are used for averageair velocities at the above locations I. 2, 3 and 4, respectively.Turbulence intensity measurements (I) were made as describedhereinbefore at locations 1, 3 and 4. The velocity variations in theregion of the air stream which advances into the fiber path was alsomeasured at location 1 (AV/V).

Without making any changes in the operation of the apparatus except todecrease the speed of the web'- collecting screen, a series of webs withabout 3 oz./yd'. unit weight is also produced.

The blotchiness of the l oz./yd. and 3 oz./yd.- webs is measured bymeans of a Paper Formation Tester (M. N. Davis et .11., TechnicalAssociation of the Pulp and Paper Industry, Technical Papers, Series l8.386-39] 1935 As a standard for determination of formation value (FV), asuitable number of sheets of l oz./yd. onion-skin paper are used to givea unit weight corresponding to the web samples to be examined. The data'reported in the table are averages of three determinations on singlesamples of each web.

TABLE 1 Incoming Air Stream Over Disperser Path to Collector Run g No.Mixing Stream Generator V, \\'/W V,/\/,, I V. V l, V, I.

a None 8.700 1 4 0.75 1.4 13.600 1.17 1 1.700 1.7 i 9.000 1.2 bl/l6-inch diameter \\'irc* 8.700 1 4 0.75 26 13.600 1.17 1 1.700 2.09.000 1.8 c 3/32-inch diameter wire* 8.700 3 0.75 2.9 13.600 1.17 11.700 2.8 9.000 2.0 d l/8-inch diameter wire* 8.600 1 3 0.74 3.3 13.5001.16 1 1.600 3.3 8.900 2.2 e 3/l6-ineh diameter rod* 8.600 2 0.74 6.213.500 1.16 1 1.600 4.5 8.900 4.2 f l/4-irlch diameter rod" 8.500 3 30.73 8.3 13.400 1.15 11.500 5.6 8.800 4.8 g l-inch gate opening 8,200 70.71 I 3.3 12,800 1.10 1 1.100 13.5 8.500 7.2 h 3/4-inch gate opening" 18.000 -5 14 0.69 25.1 12.600 1.08 10,900 19.5 8.300 12.1

Notes The wire or rod mixing stream generator is located across themiddle of the incoming air stream at location X. The gate is at locationX on the sitle ol' the duct op osite the disperser roll (i.e.. the gateopening is on the disperser-roll side of the duct).

V,, lnitial projection velocity oi the lihers. l'L/niin. V Averagevelocity of incoming air stream. l'L/min.

AV/W Velocity variation across the \\'1L1lh ol" the region of the airstream advancing into the path of liber llo\v. 'Hlhot o1 \vidth.Velocity \ariation in the region of the air stream advancing into thepath of fiber flow. 1").

l= 'l'urhulenee intensity z '1. Suhscripts refer to the specifiedlocations 1. 2. 3 and 4.

FORMATION VALUE The webs made under the conditions described in thetable above for runs a. I), c. (1 and 0 were of excellent to goodqualityhaving very little blotchiness. The websof run f; although stillsatisfactory. were more blotchy. Webs of run g. although made within thescope of the invention. were still more blotchy and somewhat marginal inquality. Run I! produced the blotchiest and most streaky web in thisseries and is considered to be unsatisfactory for the purposes of thisinvention.

From this series of runs and others it is concluded that low, stableturbulence intensities of less than 15% and preferably less than 7%along the fiber stream path arerequired to produce high quality webs athigh rates. Further. a velocity variation of less,than i1571 in theregion of air flow that advances into the fiber path is also requiredand a variation of less than 12107: is preferable.

In a second series of runs without mixing stream generators thevelocities ratios V,/V,, and Vg/V were varied between 0.4 and 2.0 andbetween 0.6 and 3.]. respectively. This series of tests and othersshowed that the suitable range for V /V is between about 0.5 and 3.5.with 0.5 to 1.2 being preferred. and the suitable range for V is between0.5 and 3.0. with 0.7 to 1.7 being preferred in order to obtain the mostuniform webs at the highest production rates. In addition. these testsshowed that the suitable range for the average air velocity at. anycross-section downstream of the location where the thin fiber streamforms is between 0.25 and 3 times and preferably between 0.4 and 1.2times the initial average air velocity (V EXAMPLEV Good qualitydispersions (as judged by high-speed photographs of fiber flying overthedisperser roll or by inspection of thin webs laid down from thedispersion) suitable for making a uniform 1 ounce per square yard web.with no fiber clumps or neps and substantially no breakage of fibers areobtained. when using the conditions given in Table I1.

The fiber for Run No. l consistsofa mixture of 65% 1 inch long. 1.5 dpfacrylic. fibers and 35% /2 inch long. 1.5 dpf rayon fibers. The fiberfor Runs 2 to 5 is 1.5 dpf rayon with a length of 1 inch for Run 2 and0.75 inchfor Runs-3 to 5. I

Batts of well-opened fibers made by cross-lapping a card web in adirection transverse to the dispersion feeding direction arefed to anapparatus similar to vFIG. 7 using a 16 inch diameter disperser roll,operating at a peripheral speed of 12,500 feet per minute.

The throughput rate of Runs 3 to 5 is the maximum possible under theconditions used which will produce an acceptable degree of dispersion.However. the use of a 60 oz./yd. feed batt under the conditions of Run 3gives a better dispersion and an 8.0 o7..-/yd. feed batt gives even alarger improvement;-

When webs of from about 1 .to .4 oz./yd. weight are made from the abovefiber.dispersions. using the air stream conditions of Example 1. uniformwebs are made.

What ls Claimed ls:

1. In an apparatus comprising duct means for conveying fibers in astream of air, air supply means for directing a stream of air throughthe duct means, a rotating toothed disperser roll and coactingstationary disperser plate for projecting fibers through an opening inthe duct means to form a stream of fibers in the stream of air, andcondenser means for separating the fibers from the air to form a web;the improvement for high speed production of uniform webs whichcomprises, in the air supply means, a highly uniform air passage with alarger cross-sectional area than said duct means connected directly tothe duct means by a converging section in the form of a smooth, gradualcurve, in combination with screens and a honeycomb structure located inthe larger air passage to provide a uniform flow substantially free ofturbulence and vorticity.

2. Apparatus as defined in claim 1 wherein said duct means has asubstantially uniform rectangular crosssection along the path of thefiber stream to said condenser means.

3. Apparatus as defined in claim 1 wherein said duct means has asubstantially uniform crosssection up to the opening at said disperserroll and thereafter reduces in cross-sectional area to cause the air toaccelerate after the fiber stream is formed 4. Apparatus as defined inclaim 1 wherein boundary layer control means is incorporated in the ductmeans prior to the opening at said disperser roll for improving theboundary layer of air through which the fibers are projected.

5. Apparatus as defined in claim 4 wherein said boundary layer controlmeans is a thin boundary obstruction extending across the duct walls toeliminate streamwise vorticity in boundary air flow.

6. Apparatus as defined in claim 4 wherein said boundary layer controlmeans is a suction slot close to the opening at the disperser roll forremoving turbulent boundary air flow.

20 7. Apparatus as defined in claim 1 wherein eddy generator meansextends across the duct means prior to the opening at the disperserroll.

l l =l l

1. In an apparatus comprising duct means for conveying fibers in astream of air, air supply means for directing a stream of air throughthe duct means, a rotating toothed disperser roll and coactingstationary disperser plate for projecting fibers through an opening inthe duct means to form a stream of fibers in the stream of air, andcondenser means for separating the fibers from the air to form a web;the improvement for high speed production of uniform webs whichcomprises, in the air supply means, a highly uniform air passage with alarger cross-sectional area than said duct means connected directly tothe duct means by a converging section in the form of a smooth, gradualcurve, in combination with screens and a honeycomb structure located inthe larger air passage to provide a uniform flow substantially free ofturbulence and vorticity.
 2. Apparatus as defined in claim 1 whereinsaid duct means has a substantially uniform rectangular cross-sectionalong the path of the fiber stream to said condenser means.
 3. Apparatusas defined in claim 1 wherein said duct means has a substantiallyuniform cross-section up to the opening at said disperser roll andthereafter reduces in cross-sectional area to cause the air toaccelerate after the fiber stream is formed.
 4. Apparatus as defined inclaim 1 wherein boundary layer control means is incorporated in the ductmeans prior to the opening at said disperser roll for improving theboundary layer of air through which the fibers are projected. 5.Apparatus as defined in claim 4 wherein said boundary layer controlmeans is a thin boundary obstruction extending across the duct walls toeliminate streamwise vorticity in boundary air flow.
 6. Apparatus asdefined in claim 4 wherein said boundary layer control means is asuction slot close to the opening at the disperser roll for removingturbulent boundary air flow.
 7. Apparatus as defined in claim 1 whereineddy generator means extends across the duct means prior to the openingat the disperser roll.